The present invention relates to a magnet assembly suitable for use in an MRI (Magnetic Resonance Imaging) apparatus, and more specifically to a magnet assembly for an MRI apparatus, which is capable of suitably suppressing the influence of temperature on a static magnetic field.
Conventional techniques according to MRI apparatus, for suppressing the influence of temperature on static magnetic fields have been disclosed in Japanese Patent Application Laid-Open Nos. 63-43649 and 63-278310, for example.
According to the MRI apparatus disclosed in Japanese Patent Application Laid-Open No. 63-43649, a heat insulating material is placed on an inner wall of an enclosure for integrally covering a lower base yoke, columnar yokes and an upper base yoke so that they are thermally cut off from the surroundings. Further, an air flow path is defined between each yoke and the heat insulating material so that air is allowed to forcedly flow therethrough by a fan. Furthermore, the temperature of the air is adjusted by a temperature adjusting heater and a temperature sensor.
According to the MRI apparatus disclosed in Japanese Patent Application Laid-Open No. 63-278310, sheet-like startup heaters are respectively placed on an upper surface of an upper base yoke and a lower surface of a lower base yoke. Upon startup of the MRI apparatus, the yokes are heated by the startup heaters. After startup of the MRI apparatus, the startup heaters are de-energized and the temperature of air existing around the yokes is adjusted by a temperature adjusting heater and a temperature sensor just in the same as the MRI apparatus disclosed in Japanese Patent Application Laid-Open No. 63-43649.
Both the cases of the two prior arts, the yokes are surrounded by the heat insulating material. The air flow path is defined between the yokes and the heat insulating material so that the air is allowed to forcedly flow therethrough by a fan. Further, the temperature of the air flowing through the air flow path is adjusted by the temperature adjusting heater and the temperature sensor to provide steady temperature control, whereby the influence of temperature on the static magnetic field is suppressed.
However, a problem arises that since the yokes are heated via the air, the efficiency is poor. Further, a problem also arises that a control follow-up characteristic is degraded.
Further, according to the MRI apparatus disclosed in Japanese Patent Application Laid-Open No. 63-278310, the sheet-like startup heaters are respectively placed on the upper surface of the upper base yoke and the lower surface of the lower base yoke.
However, a problem arises that if the heaters are mounted on the upper surface of the upper base yoke, heat is dissipated upward and hence thermal efficiency becomes poor. Further, a problem also arises that the attachment of the heaters to the lower surface of the lower base yoke provides bad workability.